In this particular case, though, the plant germinates all year and flowers a few months later, so I don't think it falls into any other category than "annual." I know this because it's highly invasive in my garden and I have to pull hundreds out all year instead of just once or twice a year. I'm always tempted to keep a few in the locations where nothing else will grow, however, and it reseeds so profusely that I'll never be completely rid of it.
